The present invention relates to a method for reducing natural system oscillations with respect to a ground potential in an electrical drive having a voltage intermediate-circuit converter with a controlled input converter and with an input-side inductance, namely a mains system input inductor using the step-up controller mode, and having an electric motor connected thereto, for example a motor using field coil technology, and to a corresponding electrical drive and intermediate-circuit converter voltage.
In present-day converter systems with a intermediate circuit voltage, e.g., in multi-shaft converter systems, system oscillations can be formed which are virtually undamped. This relates primarily to converters having a voltage intermediate circuit and having a controlled feeder in the form of a regulated mains-system-side converter, which also referred to as an input converter.
Converters are principally used for operating electrical machines at a variable supply frequency. An intermediate circuit frequency converter allows an electric motor, for example in a three-phase machine such as a synchronous machine, no longer to be operated directly from the mains system and hence at a fixed rotation speed, since the fixed mains system can be replaced by an electronically produced, variable-frequency and variable-amplitude mains system for supplying the electrical machine.
The two mains systems, first the supply mains system, where the amplitude and frequency are fixed, and second the mains system supplying the electrical machine where the amplitude and frequency are variable, are decoupled via a DC voltage store or a direct current store in the form of an intermediate circuit. Such intermediate-circuit converters in this case essentially have three central assemblies:
a mains-system-side input converter, which can be designed to be uncontrolled (for example diode bridges), or controlled, in which case energy can be fed back into the mains system only when using a controlled input converter;
an energy store in the intermediate circuit in the form of a capacitor in a voltage intermediate circuit and an inductor in a current intermediate circuit; and
an output-side machine converter or inverter for supplying the machine, which generally uses a three-phase bridge circuit having six active current devices which can be turned off, for example IGBT transistors, to convert the DC voltage in a voltage intermediate circuit into a three-phase voltage system.
Such a converter system with a voltage intermediate circuit which is preferably used, inter alia, for main drives and servo drives in machine tools, robots and production machines owing to its very wide frequency and amplitude control range, is shown in the form of an outline sketch in FIG. 1.
The converter UR is connected via a filter F and an energy-storage inductor, whose inductance is LK, to a three-phase mains system N. The converter UR has a feeder E, a voltage intermediate circuit with the energy-storage capacitance CZK, and an output inverter W. FIG. 1 shows a regulated feeder E, which is operated in a controlled manner by means of switching components (for example a three-phase bridge circuit composed of IGBT transistors), as a result of which the arrangement as shown in FIG. 1 experiences a stimulus A1. The inverter W is likewise controlled via further switching components, for example by means of a three-phase bridge circuit having six IGBT transistors. The fact that switching operations also take place in the inverter likewise represents a stimulus A2 to the system. The capacitor CZK in the voltage intermediate circuit is connected between the positive intermediate circuit rail P600 and the negative intermediate circuit rail M600. The inverter is connected on the output side via a line LT and by means of a protective-ground conductor PE and a shield SM to a motor M, in the form of a three-phase machine.
The fixed-frequency three-phase mains system N feeds the intermediate circuit capacitor CZK via the filter F and the energy-storage inductor LK by means of the regulated feeder and via the input converter E, with the input converter E (for example a pulse-controlled converter) operating together with the energy-storage inductor LK as a step-up controller. Once current has flowed through the energy-storage inductor LK, it is connected to the intermediate circuit and forces the current against the greater voltage into the capacitor CZK. This also allows the intermediate circuit voltage to be kept above the peak value of the mains voltage.
This combination thus effectively represents a DC voltage source. The inverter W uses this DC voltage to form a three-phase voltage system in which, in contrast to the sinusoidal voltage from a three-phase generator, the output voltage does not have an ideal sinusoidal oscillation profile, but also has harmonics since it is produced electronically via a bridge circuit.
In addition to the above-described elements in such an arrangement, it is necessary to remember that parasitic capacitances occur which assist the formation of system oscillations in such a converter system. For example, in addition to the filter F with a discharge capacitance CF, the input converter E, the inverter W and the motor M all have discharge capacitances CE, CW and CM to ground. Furthermore, the line LT has a capacitance CPE to the protective-ground conductor PE, and a capacitance CSM to the grounded shield SM.
It has now been found that these system oscillations are stimulated in a particularly pronounced manner in the feeder E. Depending on the control method chosen for the feeder, two or three phases of the mains system N are in this case short-circuited, in order to cause current to flow through the energy-storage inductor LK. If all three phases U, V, W are short-circuited, then either the positive P600 or the negative intermediate circuit rail M600 is rigidly locked to the star point of the supply mains system (generally close to ground potential depending on the zero system component). If two phases of the mains system N are short-circuited, then the relevant intermediate circuit rails P600 and M600 are rigidly locked to an inductive voltage divider from the two mains system phases.
Depending on the mains voltage situation, this voltage is close to ground potential (approximately 50-60 V). Since the intermediate circuit capacitance CZK is generally large (continuous voltage profile), the other intermediate circuit rail 600 V is lower or higher, and may thus also drag down the remaining mains system phase. In both situations, the intermediate circuit is particularly severely deflected from its xe2x80x9cnaturalxe2x80x9d, balanced rest position (xc2x1300 V with respect to ground), thus representing a particularly powerful stimulus to system oscillation.
With regard to the production of undesirable system oscillations, the frequency band which is relevant for the application area of less than 50 to 100 kHz allows a resonant frequency to be calculated with concentrated elements. In this case, the discharge capacitances CF to ground in the filter F are generally so large that they do not govern the frequency. In this case, it can be assumed that there is a dominant stimulus to oscillations before the described capacitances, and the filter discharge capacitance CF can be ignored.
The resonant frequency fres(sys) of this system, which is referred to by fsys in the following text, thus becomes:                               f          sys                =                  1                      2            ⁢            π            ⁢                                                            L                  ∑                                ·                                  C                  ∑                                                                                        (        1        )            
where
L"Egr"=LK+LFxe2x80x83xe2x80x83(2)
where LK represents the dominant component and LF the unbalanced inductive elements in the filter (for example current-compensated inductors) which act on the converter side, and
C"Egr"=CE+CW+CPE+CSM+CMxe2x80x83xe2x80x83(3)
This expression is shown schematically in FIG. 2. In this case, L"Egr" and C"Egr" form a passive circuit, which is stimulated by a stimulus A and starts to oscillate at its natural resonant frequency fsys. As a consequence, the potentials on the intermediate circuit rails P600 and M600 are modulated, in addition to the shift with an amplitude of 600 V, for example, resulting from the operating procedure, with an additional undesirable oscillation at an amplitude of up to several hundred volts.
In electric motors M in general, but particularly when they are designed using field coil technology (for example torque motors), a frequency response with pronounced resonant peaks with respect to ground potential can occur if they are stimulated in the common mode with respect to ground at all the motor terminals, for example by the undesirable system oscillations described above.
These resonance points can be explained by an unbalanced equivalent circuit formed by a lattice network circuit K with parasitic elements (inductances L and discharge capacitances C) in the motor winding, as is shown schematically in FIG. 3. In this case, the winding section for one phase U of a three-phase motor M having the three phases U, V, W is shown by way of example, and in this case the winding sections are electrically connected to one another at the motor star point S. The input voltages of the three-phase current generated by the inverter W are applied to the outer terminals of the respective winding sections opposite the star point S.
This relates in particular to motors using field coil technology, in which individual lattice four-pole networks of the lattice network K are macroscopically plausible by virtue of the design, and essentially correspond to an individual field coil. With field coil technology, the magnetic cores, which are composed of electrical laminations, have teeth which act as pole cores, onto which prefabricated coils are placed and wired-up as appropriate. As can be seen in FIG. 3, the individual inductances L are electrically connected in series, with each field coil having a capacitive coupling to the pole core (electrical lamination), on which the coil is mounted. These respective capacitances are shown as discharge capacitances C to ground, and are formed by the magnetic core.
However, the above-described phenomenon can also be explained for motors having a different configuration (for example using what is referred to as a wild winding) by a model of a lattice network K, since this represents an equivalent circuit with identical four-pole networks in the form of LC tuned circuits, with the elements simulating the frequency response. In this case, the peak occurs in the region of the star point S, which is normally not deliberately subjected to voltage loads. If the system oscillation of a converter system is near the motor""s natural frequency, then the insulation to ground, in particular at the star point S, can be overloaded, leading to premature failure of the motor M, since the resonance results in considerably greater voltages at the motor star point than those which can occur at the motor terminals.
This condition applies in principle to all voltage levels (low-voltage, medium-voltage and high-voltage systems), but particularly when the step-up controller principle (with an energy-storage inductor LK) is being used on the converter side UR, and a frequency response with pronounced resonant peaks with respect to ground potential occurs on the other side in the motor M, for example in motors with a particularly low motor natural frequency. In this case, the intrinsic damping in the motor, resulting from any eddy current losses, remagnetization losses etc, is particularly low.
The object of the present invention is thus to avoid such natural system oscillations in a converter system.
Accordingly, the object of the present invention is achieved by a method for reducing natural system oscillations with respect to the ground potential in an electrical drive having a voltage intermediate-circuit converter with a controlled input converter and with an input-side inductance, in particular a mains system input inductor, and having an electric motor connected thereto, and preferably one using field coil technology. In such an application, the voltage intermediate circuit is periodically disconnected from the supply mains system at times which are synchronized to triggering equipment for the input converter. Furthermore, if the controlled input converter having the input-side mains system input inductor operates on the step-controller principle, it is recommended that the voltage intermediate circuit be disconnected from the supply mains system periodically at times which are synchronized to triggering equipment for the input converter. Where the input converter is controlled in the square-wave current mode, then it has been found to be advantageous to always disconnect the voltage intermediate circuit from the input converter as long as current is flowing in the input-side inductance.
If, on the other hand, the input converter is controlled using a sine-weighted pulse pattern, then the voltage intermediate circuit is preferably always disconnected from the input converter for as long as it is switching zero vectors. If the input converter is operating with general space-vector modulation, then the voltage intermediate circuit is advantageously always disconnected from the input converter for as long as it is switching zero vectors.
When the square-wave current mode or sine-wave current mode is being used to drive the input converter, the decoupling of the voltage intermediate circuit according to the invention allows the voltage intermediate circuit to be balanced with respect to ground potential via Y capacitors.
Switching power semiconductor switches, in particular IGBT transistors, have been found to be advantageous for this periodic disconnection of the voltage input circuit from the input converter. Particularly good results can be achieved if two phases of the voltage intermediate circuit are disconnected from the input converter. This is achieved, for example, by connecting an IGBT transistor in each supply line, between the input converter output and the two connections of the intermediate circuit capacitor.
Furthermore, the aforementioned object of the present invention is achieved by an electrical drive having a voltage intermediate-circuit converter with a controlled input converter and with an input-side inductance, in particular a mains system input inductor, and having an electrical motor connected thereto, in particular a motor using field pole technology. This is achieved by at least one switching means for periodic disconnection of the voltage intermediate circuit from the supply mains system. This has been found to be particularly advantageous for a voltage intermediate-circuit converter with a controlled input converter and with an input-side mains system input inductor for operation on the step-up controller principle. It has also been found to be advantageous if each switching means can be synchronized to triggering equipment for the input converter for periodic disconnection of the voltage intermediate circuit from the input converter, for example if each switching means is used for synchronized decoupling of the voltage intermediate circuit from the input converter during suitable times for current to flow to the mains system input inductor.
According to one advantageous refinement of circuit arrangements according to the invention, IGBT transistors with respective freewheeling diodes are arranged in the supply lines between the input inductor and the voltage intermediate circuit as switching means for periodic disconnection of the voltage intermediate circuit from the input converter, with the diodes being arranged back-to-back in parallel.
As long as pronounced motor resonance points are well above any possible system oscillations of the converter system, the risk of resonant peaks at the motor star point is low. However, this situation changes the closer such resonant frequencies in the frequency response of the motor with respect to ground potential come into the area of such system oscillations by the converter system. This is due primarily to the physical size of the motor itself. The size of a motor is governed by the slot area which, for its part, acts on the capacitance CM of the motor with respect to ground potential in such a way that the discharge capacitance increases with the size of the slot area. As the discharge capacitance CM of the motor increases, the pronounced resonant frequency fres of the amplitude/frequency response of the motor with respect to ground potential falls, and thus comes closer to the area of undesirable system natural frequencies fsys of the converter system. Hence, as the geometric size of the motor increases, for example the physical length or the diameter, pronounced resonant frequencies come closer to this critical region, and the problem of resonant peaks increases.
The present invention actively and effectively counters this by providing a means to prevent the formation of such undesirable natural system oscillations fsys. The invention thus results in a considerable amount of smoothing of the converter system with respect to ground potential PE, since the potential of the intermediate circuit is no longer severely and periodically dragged down to ground.